Liquid electrophotographic developer containing isocyanate compounds



United States Patent 3,383,316 LIQUID ELECTROPHOTOGRAPHIC DEVELOPER CONTAINING ISOCYANATE COMPOUNDS Josef Matkau, Malvern, South Australia, Australia, assignor to Research Laboratories of Australia Pty. Limited, North Adelaide, South Australia, Australia No Drawing. Filed Oct. 23, 1964, Ser. No. 406,150 Claims priority, application Australia, Oct. 28, 1963, 36,938/63 5 Claims. (Cl. 252-621) This invention relates to electrophotography and more particularly to improved methods of and means for rendering visible in the reverse sense light or other radiation pattern images formed on electrophotographic surfaces and on insulating surfaces such as are known in electrostatic printing.

In one type of electrophotography a relatively conductive backing member such as metal or paper sheet having deposited thereon a photoconductive material, such as for instance vitreous selenium or a particulate photoconductor such as for instance zinc oxide in a resin binder commonly known as an electrophotographic or Electrofax layer, or an organic photoconductor, is first subjected to a corona discharge whereby a uniform electrostatic charge is deposited on to the photoconductive layer. Such charged photoconductive layer is then exposed to a light or X-ray pattern whereby the irradiated areas become discharged whereas the shielded areas remain charged and thus form a latent electrostatic image. Such latent electrostatic image is then rendered visible by applying a dry developer powder or a liquid developer comprising toner material suspended in an insulating carrier liquid in order that such developer may be attracted and held electrostatically to the charged areas. The developed image may be fixed to the surface of the photoconductor or it may be transferred to another surface and fixed thereon, if desired.

In another type of electrophotography the uncharged photoconductive surface is subjected to a radiation pattern whereby the resistivity of such photoconductor is changed in proportion to the intensity of the incident radiation and whereby a latent conductivity pattern in the photoconductive layer is formed. Such latent conductivity pattern in the photoconductive layer can be then rendered visible by dry developer methods as described in US. Patent No. 2,976,144 of A. Rose and in the article, A Review of Electrofax Behaviour, by I. A. Amick published in the RCA Review of December 1959, or other methods known in the art. We have also found that it is possible to render such conductivity patterns visible especially in electrophotographic layers employing liquid developers which possess certain characteristics that is to say which are highly sensitive to minor variations in the constitution of the photoconductor surface elfected by exposure to radiation.

As referred to in the foregoing, in the liquid developing process it is customary to prepare a liquid developer for electrostatic images by dispersing some particulate toner material such as pigment with an oil or resin or an oleoresinous varnish in an insulating carrier liquid, the polarity of such particulate matter in respect to the latent electrostaic image to be developed being determined by the nature of the materials selected for the developer or by so-called polarity control or stabilizing agents, as for instance described in the US. Patents Nos. 2,907,674 of K. A. Metcalfe et al. and 3,076,722 of H. G. Grieg.

While it is possible to produce excellent liquid developers in accordance with the prior art knowledge for the facsimile reproduction of radiation patterns on electrophotographic surfaces, it has not been possible heretofore to prepare satisfactory reverse liquid developers, that is developers containing particulate matter suspended in an insulating liquid which particulate matter is capable of depositing on to the discharged areas, that is to say, on to areas substantially free from negative electrostatic charges and of being repelled from the areas retaining a negative electrostatic charge with the result of producing a reversal reproduction such as for instance a positive copy of negative film or the like on the electrophotographic surface.

Prior art reverse toners do not deposit uniformly on large areas and such toners which container sufiiciently fine particles for continuous tone reproduction lack stability in storage. An added disadvantage of such prior art reverse toners is that only a certain limited number of pigments will respond to the reversing polarity control substances as known in the prior art whereas most of the pigments suitable for the production of four colour images resembling lithographic prints have a strong tendency to deposit on to negatively charged areas and cannot be reversed in their habit of deposition by the application of the prior art control agents for reverse developers.

A further disadvantage of the prior art liquid reverse developers is that in cases where the reverse control agent is an oil such as for instance linseed oil, the resulting image is not fixed immediately upon evaporation of the carrier liquid to the surface and this disadvantage cannot be readily overcome in the prior art processes by the incorporation of resins as so-called fixing agents because such resins may afiect the polarity of the developer and result in the developing of the charge holding areas. For similar reasons it also has not been possible to produce heretofore satisfactory liquid developers containing toner material which is capable of depositing on to image areas carrying a positive electrostatic charge as are contained on layers comprising photoconductors such as for instance selenium or lead monoxide.

Accordingly, it is a general object of this invention to provide improved compositions of matter for developing images produced by electrophotographic processes.

A further object of this invention is to provide improved compositions of matter in liquid developers for the purpose of rendering visible in the reverse sense images produced by electrophotographic processes.

A still further object of this invention is to provide improved compositions of matter for producing liquid developers containing pigments of any desired colour capable of rendering visible in the reverse sense images produced by electrophotographic processes.

Another object of this invention is to provide improved compositions of matter which by incorporation in conventional oleoresinous liquid developers render such capable of developing in the reverse sense images produced by electrophotographic processes.

Still another object of this invention is to provide improved means for preparing liquid developers which are stable in storage and which are capable of rendering visible in the reverse sense images produced by electrophotographic processes.

Yet another object of this invention is to provide improved means for preparing liquid developers which are capable of rendering visible in the reverse sense images produced by electrophotographic processes wherein such images become fixed immediately upon evaporation of the developer carrier liquid.

A further object of this invention is to provide improved means for preparing liquid developers which are high y sensitive to variations in the surface of photoconductors affected by exposure to radiation and which are thus capable of rendering visible conductivity patterns formed in photoconductive layers.

A still further object of the invention is to provide improved means for preparing liquid-developers which are capable of rendering visible a latent electrostatic image formed by positive electrostatic charges and contained on photoconductive layers.

A still further object of the invention is to provide improved means for preparing liquid developers which are capable of rendering visible a latent electrostatic image formed by positive electrostatic charges and contained on photoconductive layers.

The foregoing objects and other advantages are accomplished in accordance with this invention it having been now found that an improved composition of matter containing the isocyanate group such as present in isocyanates and polyisocyanates, such as for instance diisocyanates, when incorporated with toner materials in liquid developers in a manner as Will be described in the following renders such toner material capable of being repelled by or from areas carrying a negative electrostatic charge as contained on a photoconductive surface such as for instance on Electrofax layer whilst depositing on to the areas substantially free from such negative charge and also capable of being attracted by positive electrostatic charges forming a latent electrostatic image.

In carrying this invention into effect it is now proposed to incorporate the isocyanate compound into the liquid developer in such manner that the isocyanate compound is present in such form that it can readily react or link with or adsorb on to oils or resins or varnishes or pigment or some other particulate matter which is present in suspension as toner material in the carrier liquid. This can be achieved for instance by first blending or reacting an isocyanate compound with a disp rsing medium such as an oil or resin or an oleoresinous varnish and subsequently dispersing a pigment of the desired colour in such medium to form a toner concentrate and such concentrate is subsequently suspended in an insulating carrier liquid to form the liquid developer.

Alternatively, a toner concentrate can be first prepared by milling a pigment or some other particulate matter with a dispersing medium such as an oil or resin or varnish to form a conventional toner concentrate and the isocyanate compound in its natural form or in the form of a suspension or solution in a solvent or oil or resin or varnish can be subsequently added to such toner concentrate prior or after such concentrate is suspended in the carrier liquid to form the liquid developer.

It is also possible to disperse the desired pigment or some other particulate matter directly in an isocyanate resin or compound or in a solution of such and such pigment dispersion can be suspended in an insulating liquid to produce the liquid developer or such pigment dispersion can be used in combination With conventional toner concentrates or with oils or resins or varnishes in a liquid developer; it is also possible to dry such pigment dispersion and then mill the dry material to obtain a powder comprising pigment particles coated with isocyanate compound. Such powder can be suspended in an insulating carrier liquid to produce a liquid developer or such powder can be blended with oils or resins or varnishes or conventional toner concentrate and subsequently suspended in an insulating carrier liquid to form a liquid developer.

The isocyanate compound incorporated into a liquid developer in accordance with the methods described in the foregoing surprisingly renders the toner material in such liquid developer capable of depositing on to a latent electrostatic image formed by positive electrostatic charges and also capable of being repelled from latent image areas formed by negative electrostatic charges such as for instance contained on electrophotographic layers whilst such toner material is also capable of depositing on to the discharged non-image areas on such electrophotographic layers whereby a reverse reproduction of a radiation pattern can be obtained. The principles governing this effect caused by the presence of isocyanate or poly isocyanate groups are not yet fully understood. Whilst it is not desired to be bound by a theory, it is however possible that such effect could be due to preferential adsorption of large molecules or molecular chains or groups containing isocyanate or polyisocyanate groups on to the pigment particles themselves thereby affecting the normal or natural orientation of such particles suspended in an insulating carrier liquid in the presence of an electrostatic field or it is also possible that the isocyanate or polyisocyanate groups by reacting or linking with the monoor dior triglyceride groups contained in oils and/ or with hydroxyl groups contained in esters of oils and resins with the resulting formation of urethane groups affect the orientation of such oils and resins suspended in an insulating carrier liquid or affect the manner in which such oils or resins would normally adsorb on to the pigment particles or other particulate matter suspended in an insulating liquid whereby the polarity of such particulate matter in the presence of an electrostatic field becomes affected by the orientation of the adsorbed matter.

It has been found that some pigments, oils, resins and varnishes have a much stronger natural tendency than other such materials when suspended in an insulating liquid to deposit on to areas containing a negative electrostatic charge. It has been also found that isocyanate compounds adsorb on to some pigments more readily than to other pigments and it has been also found that some oils or resins or oleoresinous varnishes can be reacted or blended or compounded with isocyanates more readily than other such materials. It has been also found that various types of isocyanate or polyisocyanate compounds possess various degrees of etfectiveness, probably due to the various percentage of free isocyanate or polyisocyanate groups present, and such compounds also possess various degrees of compatibility with different types of oils or resins. These variations in the nature of the isocyanate compounds and in the nature of pigments, oils, resins, oleoresinous varnishes and insulating liquid dispersants used for the liquid developer make is necessary to select the correct type of isocyanate compound and its proportion to suit the desired ingredients of the toner. It is not possible at this state of knowledge to define suitable isocyanate compounds for all the various types of pigments, oils and resins but in the examples following this disclosure there are described as a general guide the effects which some commercially available isocyanate compounds, such as for instance used in the urethane reaction to produce resins, have on certain types of pigments, oils, resins and oleoresinous varnishes.

The following examples of toner formulations will further illustrate the manner in which isocyanate compounds can be incorporated into liquid developers and it will be realised that this invention is not restricted by the cited materials and combinations thereof and that one skilled in the art of making developers for electrophotography can utilize the teachings of this invention to produce liquid developers having incorporated isocyanate compounds therein in order to obtain the desired features as disclosed in the foregoing.

In the following examples the following terminology is used:

Electrophotographic layer used in connection with liquid developers is one containing zinc oxide dispersed in a resin binder wherein such binder in cured or hardened condition is resistant to the solvent action of the liquid dispersant. Such binder material can be a short oil alkyd, an ether resin, a polyester or the like as known in the prior art.

Liquid developer comprises a positive or negative toner dispersed in a carrier liquid.

Carrier liquid is an electrically insulating liquid of the solvent type having a volume resistivity in excess of 10 ohm centimetre and a dielectric constant of less than 3, which acts as the carrier vehicle for the toner material Suspended therein.

Negative toner is capable of depositing on to an area bearing positive electrostatic charges or in case of electrophotographic layers, which are not capable of supporting a positive electrostatic charge, such toner is capable of being repelled by image areas bearing a negative electrostatic charge whilst it is capable of depositing on to nonimage areas which are free from negative electrostatic charge due to exposure to radiation. Images developed with such toner on electrophotographic sheets are a reverse reproduction of the original radiation pattern.

Positive toner is capable of depositing on to areas bearing negative electrostatic charges such as for instance the image areas contained on a charged and exposed electrophotographic layer. The images developed with such toners are a facsimile reproduction of the original radiation pattern.

Reversing agent is a material in accordance with this invention which contains an isocyanate compound and is capable of imparting negative properties to the toner material.

Dispersing media are oils, resins and oleoresinous varnishes used to wet pigment material and aid suspension in the carrier liquid.

EXAMPLE 1 Grams Dehydrated castor oil 45 Polylite 8625 8 The blend was heated to 120 C. and kept at this temperature for five minutes. After cooling the blend was mixed with grams Microlith black CT pigment and milled. Microlith black CT is a resin dispersed carbon black pigment produced by Ciba. The milled paste was diluted with 50 grams toluol to form the toner concentrate. This concentrate suspended in a carrier liquid comprising a 1:1 mixture by volume of Shell X55 and Solvesso 100 solvents in the proportion of approximately 1 part of toner concentrate to 50 parts of carrier liquid by volume produced a negative developer. The images were found to be fixed upon evaporation of the carrier liquid. The Microlith Black CT pigment milled with the dehydrated castor oil without the addition of the isocyanate compound produced a positive developer. Shell X55 is a substantially aliphatic solvent produced by the Shell Co. Sp. Gr. 0.72, boiling range 60l55 C., aromatic content ll% and Solvesso 100 is an aromatic solvent produced by Standard Oil Co., Sp. Gr. 0.871, boiling range 160174 C., aromatic content 97%.

EXAMPLE 2 Dehydrated castor oil was blended with Desmodur L, an isocyanate solution of 75% solids in ethyl acetate, sp. gr. 1.170, produced by Bayer, in the proportion of Grams Dehydrated castor oil 60 Desmodur L 6 The blend was heated to 140 C. and kept at this temperature for 10 minutes. The precipitate formed in the blend was removed and the filtrate was mixed with grams Microlith black CT pigment and milled. The milled paste was diluted with 75 grams toluol to form the toner concentrate. This concentrate dispersed in Shell X55 solvent as the carrier liquid in the proportion of approxi mately 1 part of concentrate to 50 parts by volume of carrier liquid produced a negative developer. The images were found to be fixed upon evaporation of the carrier liquid.

EXAMPLE 3 Grams Bodied soya bean oil 60 Microlith black CT pigment 20 were milled until a smoth paste was obtained. To this paste were added with stirring 10 grams Desmodur L dissolved in 20 grams toluol.

This toner concentrate dispersed in Shell X55 solvent as the carrier liquid in the proportion approximately one part of concentrate to parts by volume of carrier liquid produced a negative developer whereas the paste containing soya bean oil and pigment only, produced a positive toner.

EXAMPLE 4 The dehydrated castor oil of Example 2 Was replaced by a bodied linseed oil. This produced a negative toner whereas a toner paste prepared by milling the linseed oil with the Microlith black CT pigment without the reversing agent Desmodur L produced a positive toner although linseed oil is claimed in the prior art as a negative polarity control agent.

EXAMPLE 5 Grams Hydrogenated rosin 5.0 Desmodur L 5.0 Toluol 20.0

EXAMPLE 6 Grams Ester gum 10.0 Toluol 20.0 Microlith black CT 5.0

were ball-milled for four hours. This toner concentrate when dispersed in the proportion of one part by volume of concentrate to 50 parts of the solvent Shell X as the carrier liquid produced a positive developer.

To the concentrate were then added Grams Desmodur L 10.0 Toluol 20.0

The Desmodur L was dissolved in the toluol prior to adding. The resulting concentrate when dispersed in the above proportion in the same carrier liquid produced a negative developer.

EXAMPLE 7 A varnish was prepared as follows:

Grams Dehydrated castor oil 20.0 Hydrogenated rosin 5.0

were heated to 250 F. were kept at this temperature for five minutes.

10.0 grams Desmodur L was then added with stirring and the temperature was raised to 290 F. for 5 minutes. Upon cooling the precipitate was removed and the filtrate was blended with 10.0 grams Microlith Bordeaux pigment and milled to form the toner concentrate. The Microlith Bordeaux is a resin dispersed pigment manufactured by Ciba. One part of this concentrate was suspended in 50 parts by volume in the carrier liquid comprising a 1:1 mixture of the Shell X55 solvent and toluol. The resulting developer was negative whereas the pigment dispersed in the varnish without the addition of the Desmodur L produced a positive developer.

7 EXAMPLE 8 Grams Polystyrene 5.0 Toluol 10.0 Microlith black CT 5.0

were ball-milled for eight hours. This toner concentrate when suspended in the proportion of one part of concentrate in 50 parts by volume in the carrier liquid comprising a solvent mixture of Shell X55 and Solvesso 100 in a one to one proportion by volume resulted in a positive developer. To the concentrate were then added 10.0 grams Polylite 8625 dissolved in 20.0 grams toluol.

The resulting concentrate when dispersed in the above carrier liquid in the same proportion produced a negative developer.

EXAMPLE 9 To the positive liquid developer of Example 8 prepared by suspending the positive toner concentrate in the carrier liquid were added Polylite 8625 solution in toluol (33% by weight) in the proportion of one part of Polylite solution to 50 parts of the liquid developer by volume. This produced a negative developer.

EXAMPLE 10 Grams Desmalkyd RS-l65 resin 60 Microlith black CT pigment were milled to produce a toner concentrate paste. Desnialkyd RS-165 is a urethane alkyd resin containing dehydrated castor oil and soya bean oil and polyisocyanate, 60% solids, manufactured by Bayer. This toner was dispersed in the proportion of one part of toner to approximately 50 parts by volume of a carrier liquid comprising any one of the following solvents: n-hexane, nheptane, cyclohexane, Shell X55, trichlorofluoroethane, Shellsol T which is an aliphatic naphtha boiling range 180206 degrees C., sp. gr. 0.766, KB value 26 produced by Shell Co., odorless mineral spirits, Solvesso 100, perehloroethylene and carbon tetrachloride. In each case a negative developer was produced. The images were found to be fixed upon evaporation of the carrier liquid. This toner can be also used for rendering visible the conductivity patterns in electrophotographic layers as referred to in the foregoing.

EXAMPLE 11 The Microlith black CT pigment of Example 10 was replaced by Prussian blue pigment.

EXAMPLE 12 The Microlith black CT pigment of Example 10 was replaced by the pigment carbon black.

EXAMPLE 13 The Microlith black CT pigment of Example 10 was replaced by the pigment phthalocyanine blue, which is a very strongly positive pigment. The resulting toner was positive but upon addition of Desmodur L in the proportion of one part of Desmodur L by weight to 2 parts of the toner paste and re-milling, a toner concentrate was produced which was negative. This paste was not dispersable in the lower KB carrier liquids such as n-hexane, but it was easily suspended in Shell X55, Solvesso 100, perchloroethylene and carbon tetrachloride.

EXAMPLE 14 Grams Desmodur L 10 Soya bean oil 10 Dehydrated castor oil 10 were mixed and heated at 180 C. for five minutes. Upon cooling the precipitate was removed and the filtrate was diluted with toluol in 1:1 proportion by weight to form a reversing agent.

8 A toner paste was produced by milling Grams Prussian blue pigment 10.0 Bodied saillower oil 20.0

This toner paste suspended in the carrier liquid Shell X55 solvent in the proportion of 1 part of paste to 50 parts by volume of the carrier liquid produced a positive developer. Two parts of the above reversing agent were added to 50 parts of the positive liquid developer and this addition resulted in a negative liquid developer.

EXAMPLE 15 In the Example 14 the Desmodur L in the reversing agent was replaced with the same quantity of Suprasec P, which is an isocyanate adduct with 11% of isocyanate value based on NCO, produced by ICI.

This produced a reversing agent which when added to the positive liquid developer of Example 14 resulted in a negative liquid developer. The images were found to be fixed upon evaporation of the carrier liquid.

Desmodur L is a polymeric adduct of toluylene diisocyanate (TDI) and a polyhydric alcohol specifically equivalent to the reaction product of 5 moles of (1) the mixture of 2,4 toluylene diisocyanate and 35% 2,6 toluylene diisocyanate with (2) 1 mole of trimethylolpropane and (3) 1 mole of l-3 or 1-4 butanediol.

What I claim is:

1. A liquid developer for rendering visible electrostaiic patterns comprising an electrically insulating carrier liquid having a volume resistivity in excess of 10 ohm cm. and a dielectric constant of less than 3 and suspended in such carrier liquid a particulate toner material capable of being attracted and deposited electrostatically, characterized in that the toner consists essentially of particles of (l) a pigment, (2) a polymeric isocyanate compound containing repeated units of the following general structure wherein the quantity by weight of the said isocyanate compound present is so proportioned in relation to the quantity of the said toner material by weight that the said isocyanate compound renders the said toner material capable of being repelled from areas carrying a negative electrostatic charge contained on an electrophotographic surface whilst depositing on to the said surface in areas substantially free from negative electrostatic charge, and (3) an oleoresinous substance selected from the group consisting of dehydrated castor oil, linseed oil, soya bean oil, ester gum and hydrogenated rosin.

2. A liquid developer according to claim 1 and wherein the quantity of weight of the said isocyanate compound is present in the proportion of from 10% to 50% by weight of the said toner material.

3. The method of preparing a toner concentrate for a liquid developer which comprises blending polyisocyanate compound containing units of the following general structure with a greater quantity of dispersing agent for a particulate toner, then milling the blend with a particulate toner to produce a paste in which the isocyanate compound is attached to the said toner particles, and then suspending the said paste in an electrically insulating hydrocarbon liquid, the proportions being about one gram of polyisocyanate compound to two to six grams of the dispersing agent, about two to four grams of the blend to one gram of the toner, and about one gram of paste to one to two grams of insulating hydrocarbon liquid, the hydrocarbon liquid having a volume resistivity in excess of 10 ohm cm. and a dielectric constant of less than three.

4. The method of preparing a toner concentrate for a liquid developer which comprises milling a dispersing agent with a particulate toner to produce a paste, then mixing into the paste a polymeric isocyanate compound containing units of the following general structure whereby the isocyanate compound is attached to the said toner particles by the dispersing agent, and then suspending the said paste in an electrically insulating hydrocarbon liquid, and in which the proportions are about one gram of toner to two to five grams of the dispersing agent, about two to four grams of the paste with one gram of the isocyanate and about one gram of paste containing the isocyanate to one gram of hydrocarbon liquid, the hydrocarbon liquid having a volume resistivity in excess of 10 ohm cm. and a dielectric constant of less than three.

5. A developer according to claim 1 in which the isocyanate compound is an adduct of toluylene diisocyanate and a polyhydric alcohol.

References Cited UNITED STATES PATENTS 2,907,674 10/1959 Metcalfe et al. 252-621 XR OTHER REFERENCES Chemical Abstracts vol. 54, col. 18982b (1960).

LEON D. ROSDOL, Primary Examiner.

J. D. WELSH, Assistant Examiner. 

1. A LIQUID DEVELOPER FOR RENDERING VISIBLE ELECTROSTATIC PATTERNS COMPRISING AN ELECTRICALLY INSULATING CARRIER LIQUID HAVING A VOLUME RESISTIVITY IN EXCESS OF 10**9 OHM CM. AND A DIELECTRIC CONSTNAT OF LESS THAN 3 AND SUSPENDED IN SUCH CARRIER LIQUID A PARTICULATE TONER MATERIAL CAPABLE OF BEING ATTRACTED AND DEPOSITED ELECTROSTATICALLY, CHARACTERIZED IN THAT THE TONER CONSISTS ESSENTIALLY OF APRTICLES OF (1) A PIGMENT, (2) A POLYMERIC ISOCYANATE COMPOUND CONTAINING REPEATED UNITS OF THE FOLLOWING GENERAL STRUCTURE 